(i)
Since Z is within the limit of applicability of the similarity solution, eq 52 can be
used to compute the rate at which heat is removed by the cooled surface:
gρl (ρl - ρv ) k 3hfgL7
q = 4.9371
Z 7/8 = 571 W .
l
l k
3W 3
(ii) The condensate rate supported by the fin is given by
q
= 2.396 10-4 kg/s = 0.86 kg/h .
m=
˙
hfg
(iii) Reading the terminal value of θ for the curve marked Z = 0.01 in Figure 8, the
dimensionless tip temperature is qt = 0.25. Thus
T - Tsat
θt = ft
= 0.25
Tfb - Tsat
Tft = 0.25 (Tfb - Tsat ) + Tsat = 43.5oC.
or
Similarly, the terminal value of ∆*1/4 for the curve marked Z = 0.01 in Figure 9 gives ∆*1/4 =
0.65. Using eq 46,
∆ = ∆*1/4 (Z)1/4 = (0.65) (0.01)1/4 0.2055 .
Invoking the definition of ∆, that is, eq 37, the film thickness δ is given by
δ = 2klL2∆ / kw = 0.0968 mm
DEHUMIDIFICATION OF AIR ON FINS
In air conditioning applications, finned cooling coils are often used to cool and dehu-
midify air. The thermal performance of these coils is not only affected by geometry,
materials and psychrometric conditions, but also by the efficiency of the fin. If the fin
temperature is lower than the dew point of air passing over the coil, then the moisture is
condensed on the fin surface and affects the fin efficiency.
This section considers the performance of fins operating in moist air streams, with
moisture condensation occurring on their surface. Simple Models discusses simple models
in which the classical fin theory for dry fins is modified to take into account the effect of
mass transfer. Conjugate Models describes two conjugate models for simultaneous heat
and mass transfer to a cooling and dehumidifying vertical rectangular fin. Experimental
studies of dehumidification in finned coil heat exchangers are covered in Experimental
Studies. The design of optimum-dimensional rectangular and triangular fins with conden-
sation is covered in Optimum Fin Design.
Simple models
Longitudinal fins
McQuiston (1975) considered moisture condensation on a longitudinal fin of rectangu-
17
Previous Page
